Well Fibrodysplasia ossificans progressiva or FOP is a genetic condition where due to a gentic malfunction the body repairs damaged muscle and connective tissue by turning it to bone. Something as simple as a deep bruise can set it off. Over the course of a life time, albeit a brief one, the persons body becomes almost completely bone.

Oddly, it made me think of someone's bones growing out of their skin, and ultimately replacing it, so that people would have a carapace of bone armor encasing their body...
Well, something like that, at least.